This invention relates generally to magnetic resonance (MR) imaging (MRI) techniques. In particular, it relates to methods for generating water/fat separated images and, more particularly, to a method for generating and identifying water/fat component MR images with reduced T2/T2* weighting.
In broad summary, the present invention relates to MRI techniques for generating images in which fat and water regions can be readily discerned. MRI is often used to non-invasively generate images of the internal organs and other body parts of human patients. It is desirable to distinguish regions of water and fat in an MR image of a patient. Distinguishing water and fat regions using MR imaging techniques is difficult. At mid-field strength the Three-Point Dixon method is the method of choice for separating water and fat images. The original Dixon method for separation of water and fat images depends upon the ability to accurately compensate for inhomogeneities in a polarizing static magnetic field BO.
A problem in using the original Dixon method occurs because of inhomogeneities in the static BO field. Prior approaches to this problem have utilized additional information obtained during single or multiple scans (each having some associated disadvantages). One such prior approach is disclosed in U.S. Pat. No. 5,909,119 (the ""119 patent) entitled xe2x80x9cMethod and Apparatus for Providing Separate Fat and Water MRI Images In A Single Acquisition Scanxe2x80x9d.
Prior approaches for separating images of water and fat have not been entirely successful in generating MR images that provide good contrast between fat and water regions. Some approaches are slow and require at least three MR scans. These approaches with long acquisition times are susceptible to motion problems that arise if the patient moves during the scan period. Approaches that use single scans (and have fast scan periods) have long minimum echo periods, which result in heavy T2/T2* weighted image contrasts. Heavy T2/T2* weighting is undesirable when strong T1 weighting is desired. Some single scan approaches, such as that disclosed in the ""119 patent, do not uniquely identify water and fat nuclei images after separation. Instead, user intervention relying on anatomical information is needed for the identification. Accordingly, there are long-felt needs for an MRI technique able to uniquely identify water and fat regions, and that provides water and fat images with reduced T2/T2* weighting. The present invention fulfills these needs.
By way of general background, MRI systems use the nuclear magnetic resonance (NMR) effects that RF transmissions at the nuclei Lamor frequency have on atomic nuclei having a net magnetic moment such as those in hydrogen. Applying RF transmissions to a patient affects the nuclear spin moments of the atomic nuclei in the body of the patient. The net magnetic moment of the nuclei in the patient are first magnetically aligned by a strong static magnetic field B0 (e.g., typically created by magnetic poles on opposite sides of the MRI imaging volume or inside a solenoidal cryogenic superconducting electromagnet). The static field B0 is altered by gradient magnetic fields created in the X, Y, and Z directions of the imaging volume. Selected nuclei, which are in spin alignment with the B0 field, are nutated by a perpendicular magnetic field of a NMR RF transmission at the Lamor frequency. The nutation causes a population of such nuclei to tip from the direction of the magnetic field B0.
As shown in FIG. 1, certain nuclei (designated by magnetic moment M0) are aligned with the xe2x80x9cZxe2x80x2xe2x80x9d axis by the static B0 field and then rotated to the Xxe2x80x2-Yxe2x80x2 plane as a result of an RF signal being imposed on the nuclei. The nuclei then precess in the Xxe2x80x2-Yxe2x80x2 plane as shown by the circulating arrow in FIG. 1 (which is a reference frame rotating at the nominal Lamor resonance frequency around the Zxe2x80x2 axis).
The NMR RF spin-nutating signal tips more than one nuclei species in the area targeted by the RF signal. Immediately after the nutating RF signal tips the nuclei, the spinning nuclei of all species are in-phase with each other. The rotating magnetic moments of all NMR species initially all rotate across the xe2x80x98Yxe2x80x99 axis all at approximately the same time. However, after the NMR nutating RF pulse ends, each species of nuclei begin to freely precess at its own characteristic speed around the Zxe2x80x2 axis.
As these nuclei precess, the phases of each of the rotating nuclei species will differ as a result of such parameters as the physical or chemical environment in which the nuclei are located. Nuclei in fat, for example, precess at a different rate than do nuclei in water. This difference in phase between water and fat nuclei is detected and used to distinguish water and fat in an MR image. In an MRI imaging pulse sequence there are also magnetic field gradients which dephase the moments due to their local resonance frequency varying in space. These phase differences in the nuclei spin moments are detected by an RF receiver and are used to determine the location and type of the source nuclei.
Once the nuclei spins are disturbed from their equilibrium, xe2x80x9crelaxationxe2x80x9d processes cause the phase-coherent component of magnetic moments in the Xxe2x80x2-Yxe2x80x2 plane to decay and the Zxe2x80x2-component to recover to its equilibrium magnitude, M0. These processes are usually characterized by exponentials whose time constants are called T2 and T1 decay times, respectively. When magnetic resonance signals are observed through flux oscillation in a plane coexistent with the Xxe2x80x2-Yxe2x80x2 plane, both of these processes decrease the signal strength as a function of time.
The relative phase of components of the magnetic moments in the Xxe2x80x2-Yxe2x80x2 plane of FIG. 1 begin aligned on the Yxe2x80x2-axis, but over time they spread out and disperse to fill the full rotational area in the Xxe2x80x2-Yxe2x80x2 plane. The nuclei of moment M2, for example, which initially crossed the Yxe2x80x2-axis at the same time as M0, gradually moves during the dephasing period to the position shown in FIG. 1 as it spins faster than M0. M1, by contrast, spins slower than both M0 and M2, and thus begins to lag them during the dephasing period. The strength of the detectable NMR response signal decays as the relative phases of the magnetic moments disperse (i.e., lose phase coherence) in the Xxe2x80x2-Yxe2x80x2 plane, a process often referred to as T2* relaxation.
Information about NMR hydrogen nuclei can be obtained, in part, by measuring their T2 and T1 decay times. In addition, before the nuclei become completely dephased, another RF signal (e.g., a 180xc2x0 signal) can tip the magnetic moments (e.g., to a 180xc2x0 inverted position). This RF signal inverts the spinning magnetic moments M0, M1 and M2 of the three species of nuclei so the fastest moment M2 now lags (instead of leading) moment M0, which in turn also now lags the slowest moment M1. Eventually, the faster moment M2 will again catch up with and pass the slowest moment M1 during which, a so-called xe2x80x9cspin-echoxe2x80x9d NMR RF response can be detected from the changes in net magnetic moment as the various magnetic moments come back into phase coherence. The whole procedure must be completed before T1 or T2 relaxation processes destroy the detectable Xxe2x80x2-Yxe2x80x2 components of the magnetic moments.
Detectable NMR RF response echoes can also be formed by application of a field gradient and the subsequent reversal of the gradient, provided that the reversal is done before T1 or T2 relaxation decays destroy MXxe2x80x2Yxe2x80x2. This is commonly called a field echo, gradient echo or racetrack echo.
The differences in the phase relationships between the species of nuclei in one tissue versus another can be used as information to separate MRI images of fat components of tissue from fluids or water-based tissue (for these purposes, xe2x80x9cwater-based tissuexe2x80x9d and xe2x80x9cfluidsxe2x80x9d are used interchangeably). Although MR images of both water and fat may contain the same or different diagnostic information, they often interfere with each other""s interpretation when overlapped in an MRI image and thus make it difficult to properly interpret the composite MR image. Somewhat different diagnostic information may also be obtained from separate MR images of only the fat-based or water-based species of NMR nuclei. At high field strengths, the separation of water and fat images or suppression of fat signals can be achieved using selective excitation or non-excitation approaches. However, at mid- or low-field strengths, approaches based on chemical shift selectivity become impractical, if not impossible. At all-field strengths, the difficulties of water/fat image separation are further exacerbated when there are large magnetic field inhomogeneities.
The difficulty in separating fat and water images in a practical MR imaging application is particularly true for mid- and low-field systems where the frequency separation between the water and fat signals is much reduced in comparison to that at high fields. The Three-Point-Dixon method has promising features for mid- or low field applications. It uses a single spin-echo sequence, but relies on the acquisition of three images for water/fat separation, an in-phase image and two out-phase images. A relatively-long scan period is required to acquire three images. Such long scan periods are susceptible to motion problems.
FIG. 2 shows the three data acquisition schemes for the three images in the Three-Point Dixon method. Slice selection is not shown for simplicity. Three different scans are used to generate three spin echo signals S0, Sxcfx80, Sxe2x88x92xcfx80. In the first scan, a 90xc2x0 pulse is followed by a 180xc2x0 pulse at a time T, yielding the spin echo S0. Then, a 90xc2x0 pulse is followed by a 180xc2x0 pulse a time T earlier than the time T, yielding a spin echo Sxcfx80. Finally, another 90xc2x0 pulse is followed by a 180xc2x0 pulse a time xcfx84 later than the time T, yielding a spin echo Sxe2x88x92xcfx80. The Dixon Methodology is described in xe2x80x9cThree-Point Dixon Technique for True Water Fat Decompositions with B0 Inhomogeneity Corrected,xe2x80x9d 18 Magnetic Resonance in Medicine, 371-383 (1991), by Glover et al., xe2x80x9cTrue Water and Fat MR Imaging With Use of Multiple-Echo Acquisitionxe2x80x9d, 173 Radiology 249-253 (1989), by Williams et al., xe2x80x9cSeparation of True Fat and Water Images By Correcting Magnetic Field Inhomogeneity In Situ,xe2x80x9d 159 Radiology 783-786 (1986), by Yeung et al., which are incorporated herein by reference, and are summarized in part below.
The value of xcfx84 (shown in FIG. 2) is determined according to xcfx84=1/(4xcex94v) with xcex94v being the frequency difference between the water and fat signals. The value of xcfx84 is chosen so the phase between the nuclei in, respectively, fat and water are: (1) in-phase, (2) out-of-phase by xcfx80, and (3) out-of-phase by xe2x88x92xcfx80. FIGS. 3a, 3b and 3c schematically show in a rotating frame the MR signals in the three different acquisition schemes. Other phase differences than xcfx80 can also be used as described in Hardy et al., JMRI, 1995. Additionally, the So signal could be derived from a gradient reversal induced field echo. It is not required that So be an RF-induced spin echo.
In a brief summary of the three point Dixon method, three NMR RF responses are required to compute separate water-based and fat-based images:
S0=a first NMR response with phase coherent fat and water NMR species;
Sxcex8=a second NMR response with a predetermined difference between fat and water NMR species in a first (e.g., xe2x80x9cpositivexe2x80x9d) direction; and
Sxe2x88x92xcex8=a third NMR response with the same predetermined phase difference between fat and water NMR species in the opposite (e.g., xe2x80x9cnegativexe2x80x9d) direction.
Once S0, Sxcex8, Sxe2x88x92xcex8 and xcex8 are known, then separate MR images of the NMR fat species and/or the NMR water species can be derived.
The following is a more specific description of an exemplary embodiment of the Three-Point Dixon method in which xcex8=xcfx80 is detailed in the Dixon paper. In the presence of field inhomogeneities, the MR signals can be described by:
S0=(Pw+Pf) 
Sxcfx80=(Pwxe2x88x92Pf)eixcfx86
Sxe2x88x92xcfx80=(Pwxe2x88x92Pf)exe2x88x92ixcfx86. 
where xcfx86 is the phase angle due to field inhomogeneities or frequency offset, and Pw and Pf are water and fat spin densities, respectively.
Thus, xcfx86 can be determined from Sxcfx80 and Sxe2x88x92xcfx80 by:
xcfx86=xc2xdarg(Sxcfx80.S*xe2x88x92xcfx80) 
where xe2x80x9cargxe2x80x9d produces the phase angle of a complex number.
Water and fat images (Iwater and Ifat, respectively) can then be reconstructed according to:
Iwater=S0+0.5Sxcfx80exe2x88x92ixcfx86+0.5Sxe2x88x92xcfx80eixcfx86
Ifat=S0xe2x88x920.5Sxcfx80exe2x88x92ixcfx86xe2x88x920.5Sxe2x88x92xcfx80eixcfx86
The phase angle xcfx86 is generally determined by phase mapping. Calculating xcfx86 from Sxcfx80 and Sxe2x88x92xcfx80 involves: (1) evaluating pixel-by-pixel the principle phase value of Sxcfx80xc2x7Sxe2x88x92xcfx80* to yield a principle phase map; (2) unwrapping the principle phase map to yield a true phase map.
In the past, water/fat separation at low and mid-level field intensities have been most successfully achieved using the above-discussed Three-Point Dixon methods. Moreover, as described in the ""119 patent, a single-scan Three-Point Dixon method (with the water and fat signals evolving a phase difference of xcfx80 during the inter-echo time (xcex94TE)) can acquire three consecutive NMR echo signals after only a single excitation pulse resulting in a significant reduction in scanning-time.
The ""119 patent describes a single-scan Three-Point Dixon imaging method to obtain the NMR raw signal data. In Three-Point Dixon imaging, a slice-selective excitation pulse is followed by the acquisition of three separate gradient-refocused signal-echoes. Each signal echo is acquired by controlling the timing and polarity of an applied read-out gradient. The time (xcex94TE) between the signal-echoes (S1, S2, S3) are selected according to the chemical-shift difference between water and fat signals, so that the two signals develop between them an angular difference of xcfx80 radians (180xc2x0) during the inter-echo time. This 180xc2x0 angular difference makes it difficult to uniquely identify fat and water images after separation, for reasons described below.
In the method described in the ""119 patent, after Fourier conversion of the raw data (k-space data) to complex frequency-domain data, also called xe2x80x9cimage-domainxe2x80x9d data, background magnetic field inhomogeneities are compensated by obtaining the compensation phase from the S1 and S3 signal-echoes using a guided region-growing phase unwrapping technique. The acquired image data are then corrected according to the compensation phase determined from S1 and S3. Water and fat signals are finally separated from the corrected image data, producing water-only or fat-only images.
The technique disclosed in the ""119 patent acquires and processes echo images in which water and fat signals are either parallel or anti-parallel, which is the so-called sampling symmetry. Thus, the three echo images acquired using the approach taught in the ""119 patent can be described as:
S1=(Wxe2x88x92F)exe2x88x92ixcfx86exe2x88x92i"PHgr"
S2=(W+F)exe2x88x92i"PHgr"
S3=(Wxe2x88x92F)e+xcfx86exe2x88x92i"PHgr"
where xcfx86 and "PHgr" are spatial dependent phase angles, S1, S2 and S3 are first, second and third NMR responses; and W and F are the water and fat echo images, respectively.
To correct for phase angles caused by static magnetic field BO inhomogeneities, the approach described in the ""119 patent devotes a great deal of effort to determine xcfx86 from arg(S3xc2x7S1*). To determine the real xcfx86 from the principle phase values, the combined phase images, P=arg(S3xc2x7S1*) is phase-unwrapped using a complex seed-oriented region-growing algorithm described in the ""119 patent.
In the approach described in the ""119 Patent and depending on the chemical composition of the seed pixel, i.e., whether it is water-dominant (W greater than F) or fat-dominant (F greater than W), the unwrapped phase image has two possibilities:
Case 1 where W greater than F:                     P        =                  2          ⁢          φ                                                  I          W                =                              S            2                    +                                    1              2                        ⁢                          S              1                        ⁢                          ⅇ                              ⅈ                ⁢                                  P                  2                                                              +                                    1              2                        ⁢                          S              3                        ⁢                          ⅇ                                                -                  ⅈ                                ⁢                                  P                  2                                                                                                      I          F                =                              S            2                    -                                    1              2                        ⁢                          S              1                        ⁢                          ⅇ                              ⅈ                ⁢                                  P                  2                                                              -                                    1              2                        ⁢                          S              3                        ⁢                          ⅇ                                                -                  ⅈ                                ⁢                                  P                  2                                                                        
Case 2 where F greater than W:                     P        =                  2          ⁢                      (                          φ              +              π                        )                                                            I          W                =                              S            2                    -                                    1              2                        ⁢                          S              1                        ⁢                          ⅇ                              ⅈ                ⁢                                  P                  2                                                              -                                    1              2                        ⁢                          S              3                        ⁢                          ⅇ                                                -                  ⅈ                                ⁢                                  P                  2                                                                                                      I          F                =                              S            2                    +                                    1              2                        ⁢                          S              1                        ⁢                          ⅇ                              ⅈ                ⁢                                  P                  2                                                              +                                    1              2                        ⁢                          S              3                        ⁢                          ⅇ                                                -                  ⅈ                                ⁢                                  P                  2                                                                        
With the approach described in the ""119 patent, water and fat signals are symmetrical in that they are either parallel or anti-parallel in all echo images. Because of this Phase symmetry, information about the difference between the precessing frequencies of water and fat nuclei is lost. Moreover, there is no other information available to automatically distinguish the water and fat signals. The descriptions in the ""119 Patent assumes that the seed pixel for phase unwrapping is water-dominant. Though water and fat images are separated, there needs to be other information. Such as anatomical differences that is used to correctly identify the separated water and fat images.
An MRI technique has been developed for distinguishing and uniquely identifying water and fat regions, and for doing so with relatively-low T2 and T2* weighting. This MRI technique breaks the sampling symmetry of prior techniques for distinguishing water and fat signals, such as is described in the ""119 patent. The sampling symmetry refers to the acquisition of image data from water and fat signals that are either parallel (in-phase) or anti-parallel (180xc2x0 out-of-phase) at the centers of all echo signals. To break the sampling symmetry, the present technique acquires two echo images with water and fat signals that are orthogonal (90xc2x0 out-of-phase) and parallel/anti-parallel, respectively. By breaking the sampling symmetry and acquiring two echo images, the present technique is able to acquire echo signals in a shorter period of time, thus reducing T2 and T2* weighting. By acquiring an echo signal in which water and fat signals are orthogonal, water and fat images can be absolutely identified according to their difference in precessing frequencies, as described later.
In one embodiment, the invention is a method of separating and identifying water and fat magnetic resonance (MR) images using an MRI apparatus, comprising the steps of: a) obtaining nuclear magnetic resonance (NMR) image data from at least a first MR echo signal and a second MR echo signal, where water and fat signals are orthogonal in the first MR echo signal and water and fat signals are either parallel or anti-parallel in the second MR echo signal; b) obtaining a field map of background main magnetic field from the second MR echo signal by applying a seed-guided phase unwrapping algorithm to the image data obtained from the second MR echo signal; c) removing the effect of the background field inhomogeneties according to the unwrapped field map to yield a corrected first MR echo signal and a corrected second MR echo signal; d) calculating an intermediate data for the corrected first MR echo image according to the sequence parameters of data acquisition; e) summing an imaginary portion of the intermediate data; f) distinguishing whether a seed reference used in applying the phase unwrapping is water-dominant or fat-dominant based on the polarity of the sum of the imaginary portion of the intermediate data, and g) determining water and fat images from the corrected first and second MR echo images based on whether the seed reference is water-dominant or fat-dominant.
In a further embodiment, the invention is a method of separating and identifying water and fat magnetic resonance (MR) images using an MRI apparatus, comprising the steps of a) obtaining nuclear magnetic resonance (NMR) image data from at least a first MR echo signal and a second MR echo signal, where water and fat signals are orthogonal in the first MR echo signal and water and fat signals are either parallel or anti-parallel in the second MR echo signal; b) obtaining a field map of background main magnetic field from the second MR echo signal by applying a seed-guided phase unwrapping algorithm to the image data obtained from the second MR echo signal according to       P    =                  1                  2          ⁢          λ                    ⁢      unwrap      ⁢              {                  arg          ⁡                      (                                          S                2                            xc3x97                              S                2                                      )                          }              ;
c) removing the effect of background field inhomogeneties according to the unwrapped field map to yield a corrected first MR echo signal and a corrected second MR echo signal; d) calculating an intermediate data for the corrected first MR echo image according to the sequence parameters of data acquisition; e) summing the imaginary portion of the intermediate data; f) distinguishing whether a seed reference used in applying the phase unwrapping is water-dominant or fat-dominant based on the polarity of the sum of the imaginary portion of the intermediate data, and g) determining water and fat images from the corrected first and second MR echo images based on whether the seed reference is water-dominant or fat-dominant.